[Patent document 1] JP-2011-045212A (US 2011/0043152 A)
Conventionally, there is known a motor drive apparatus provided with a power transducer that includes more than one semiconductor switching element. An example motor drive apparatus converts DC power from a power supply source into three-phase AC power to drive a three-phase AC motor.
According to the motor drive apparatus disclosed in patent document 1, a power line connecting the power transducer with the motor is provided with a cutoff switch that disconnects the power transducer from the motor. If the semiconductor switching element of the power transducer fails due to short-circuiting, the apparatus operates the cutoff switch (i.e., turns off the cutoff switch) to prevent the motor from entering the brake state.
Generally, the motor works as a power generator and generates a regenerative voltage if an external force rotates the rotary shaft. Basically, the power supply source and the power transducer are connected if the power transducer of the motor drive apparatus is normally driven. The regenerative voltage can be transferred to the power supply source. An external force may often rotate the rotary shaft on an electric power steering apparatus if a tire drives over an obstacle while the vehicle is running, for example. Generally, the motor drive apparatus is controlled to suppress a regenerative voltage if the motor drive apparatus is used under a condition that may often generate a regenerative voltage.
However, for example, the semiconductor switching element included in the power transducer may be short-circuited. In such a case, some motor drive apparatus turns off a switch provided between the power supply source and the power transducer to stop driving the power transducer. Such a motor drive apparatus cannot transfer a regenerative voltage to the power supply source if the regenerative voltage is generated while the power transducer stops. For example, the electric power steering apparatus cannot transfer the generated regenerative voltage to the power supply source if the power transducer fails and a tire drives over an obstacle while the vehicle is running. The motor drive apparatus cannot provide control to suppress the regenerative voltage because the power transducer stops.
There may be a motor drive apparatus that turns off the switch provided between the power supply source and the power transducer when the ignition switch is turned off. Suppose that the steering system is operated to generate a regenerative voltage while the ignition switch is turned off and the vehicle is raised using a jack, for example. The motor drive apparatus cannot transfer the generated regenerative voltage to the power supply source. The motor drive apparatus cannot provide control to suppress the regenerative voltage.
As a result, the regenerative voltage may destroy the semiconductor switching element of the power transducer. An overvoltage due to the regenerative voltage may decrease the life of a smoothing capacitor that smoothes the energy from the power supply source.
The motor drive apparatus according to patent document 1 turns off the cutoff switch between the power transducer and the motor if a short-circuiting failure occurs inside the power transducer. If the cutoff switch fails due to short-circuiting, the motor drive apparatus is equivalent to having no cutoff switch. It is impossible to protect elements such as the semiconductor switching element and the smoothing capacitor of the power transducer against a regenerative voltage.